"Moreover, even where trickle vents (small vents in windows
or bricks) are fitted, a proportion will not be used or will be
left closed," said Wilkinson, from the London School of Hygiene
and Tropical Medicine.

Radon, a gas produced from naturally occurring uranium in
soil and water, is known to increase the risk of lung cancer. It
is present in many homes in varying amounts.

The U.S. Environmental Protection Agency (EPA) estimates
radon contributes to about 21,000 lung cancer deaths each year,
mostly among people who smoke.

Using physics models, Wilkinson and his coauthors calculated
what would happen if homes in England used available methods to
become more airtight and reduced air leakiness by about one
half.

Increasing air tightness without taking any other measures
would increase indoor radon levels by an average of more than 50
percent, according to their model, described in the British
medical journal BMJ. That would mean the number of homes above
the radon action level would increase from 0.6 percent to 2
percent - corresponding to an extra 750,000 people in those
homes.

If the homes also installed purposeful ventilation methods
that worked most of the time, radon levels would still increase,
but to a lesser extent, the researchers found.

Mechanical ventilation and heat recovery systems would also
do a great deal to mitigate radon risks, unless the systems
failed.

The authors estimated current levels of radon account for
about 1,000 deaths per year in England. In their worst-case
scenario - more air tightness without extra ventilation measures
- they predicted an additional 278 radon-related deaths per
year.

"Regardless of type of home construction or perceived air
tightness of a home, the first step is to test the home, or
other structure, for radon," R. William Field said. He studies
the health effects of radon at the University of Iowa College of
Public Health in Iowa City but wasn't involved in the new study.

"Do it yourself test kits are inexpensive. If a home tests
high for radon, increasing ventilation rates and/or installing a
radon mitigation system will lower radon concentrations
substantially," Field told Reuters Health in an email.

In the U.S. there is little awareness of the dangers of
radon, he said, and only about 30 percent of homes have been
tested.

According to an EPA map, states in the Midwest and New
England tend to have the highest radon levels.

For most people, the increased risk of radon with more air
tightness would likely be small, Wilkinson said.

"These small risks add up to an appreciable burden at (the)
population level, however, if a high proportion of the housing
stock is retrofitted," he said.

Homeowners should check radon levels before and after
installing energy-efficiency retrofits, according to Wilkinson.

He suggested remediation measures like fitting extraction
pumps under the floor for homes with high radon levels. When
building a home, there are special membranes impermeable to
radon that can be built into floors, he added.